Array antennas of various types have become common for situations in which a large radiating aperture is desired, because the radiating aperture can be made up of many individual antenna elements. Array antennas are also advantageous for situations in which beam agility is desired, which is to say when the antenna beam or beams must be directed and redirected in space.
Those skilled in the arts of antenna arrays and beamformers know that antennas are transducers which transduce electromagnetic energy between unguided- and guided-wave forms. More particularly, the unguided form of electromagnetic energy is that propagating in “free space,” while guided electromagnetic energy follows a defined path established by a “transmission line” of some sort. Transmission lines include coaxial cables, microstrip and striplines, rectangular and circular waveguide tubes with conductive walls, dielectric paths, and the like. Antennas are totally reciprocal devices, which have the same beam characteristics in both transmission and reception modes. For historic reasons, the guided-wave port of an antenna is termed a “feed” port, regardless of whether the antenna operates in transmission or reception. The beam characteristics of an antenna are established, in part, by the size of the radiating portions of the antenna (the “radiating aperture”) relative to the wavelength. In the context of simple conductive antenna elements such as a monopole, dipole, or patch, the radiating aperture is viewed as being a region around the physical element. Small antennas make for broad or nondirective beams, and large antennas make for broad, narrow or directive beams. When more directivity (narrower beamwidth) is desired than can be achieved from a single antenna, several antennas may be grouped together into an “array” and fed together in a phase-controlled manner, to generate the beam characteristics of an antenna larger than that of any single antenna element. The structures which control the apportionment of power to (or from) the antenna elements are termed “beamformers,” and a beamformer includes a beam port and a plurality of element ports. In a transmit mode, the signal to be transmitted is applied to the beam port and is distributed by the beamformer to the various element ports. In the receive mode, the unguided electromagnetic signals received by the antenna elements and coupled in guided form to the element ports are combined to produce a beam signal at the beam port of the beamformer. A salient advantage of sophisticated beamformers is that they may include a plurality of beam ports, each of which distributes the electromagnetic energy in such a fashion that different beams may be generated simultaneously.
Because of cost, available volume, and weight considerations, it is often desirable to make an array antenna in the form of a planar sheet. Fabrication on planar sheets allows simultaneous manufacture of many arrayed “patch” antenna elements by methods such as printing, application of resist, and etching. Such antenna elements tend to be subject to corrosion and breakage when exposed to the elements. Consequently, the antenna elements of an array antenna are often mounted behind a protective cover or electromagnetically transparent “radome.” In the case of a planar array, the protective cover can be generally flat, so there is no need for a “dome” per se.
An array antenna, such as those used for radar purposes, may include thousands of individual antenna elements. The transmission of energy through the radome in the transmission mode of the radar tends to heat the radome, which can be disadvantageous. A radome naturally cools itself by exposure of one side to the elements. Cooling of the radome by other means is difficult, because the radome must be as transparent as possible to electromagnetic energy. Many thermally conductive elements which might be used for carrying heat away from the antenna elements and the radome are electrically conductive. Such electrically conductive materials, when located in or near the “aperture” of an antenna, tend to distort the radiation field of the antenna elements. These distortions tend to change, depending upon the direction in which the antenna beam of the array antenna is steered. This direction-dependent beam distortion makes analysis of returned signals undesirably complex.
Since the array antenna may include thousands of elemental antennas, the cost of each antenna element is an important factor in determining its suitability. An array antenna using easily-fabricated patch antennas has a radiation pattern at angles off-boresight which is the product of the pattern of an individual element and of an “array factor” which depends upon the number of elemental antennas in the array. This, in turn, means that the radiation pattern of each individual patch antenna should be spatially as broad (nondirective) as possible, so as not to adversely affect off-boresight performance of the array, and frequency-wise should tend to maintain the same beam performance over a frequency range at least as broad as that of the application to which it is directed.
Thus, the elemental antenna elements of an array are subject to limitations as far as ease of fabrication and cost, weight, off-axis directivity, heat sensitivity, and other factors such as type of feed (coax or hollow waveguide) and impedance match to the associated transmission line.
Improved or alternative array antenna elements and arrays are desired.